Perhaps they were being held back because Charlotte was not married yet; either due to the cost of them coming out or because their parents did not want Charlotte to have more competition from her own sisters; Maria is already out. This begs the question of how long would they would have kept their younger daughters at home if Charlotte had not engaged Mr. Collins to which I have no answer. I think it does highlight that Charlotte must have felt pressure to get married from within her family; it seems her younger sisters social life depended on it. ;D